Chapter 1: Chemical Reactions and Equations Exercise & In-Text Questions & Answers
1Why should a magnesium ribbon be cleaned before burning in air?
Answer & Reference:
Magnesium ribbon is cleaned to remove the layer of magnesium oxide or carbonate formed due to reaction with air, which prevents further reaction. (Page 2, Activity 1.1)
2Write the balanced equation for the following chemical reactions. (i) Hydrogen + Chlorine → Hydrogen chloride (ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride (iii) Sodium + Water → Sodium hydroxide + Hydrogen
3Write a balanced chemical equation with state symbols for: (i) Solutions of barium chloride and sodium sulphate react to give insoluble barium sulphate and sodium chloride. (ii) Sodium hydroxide reacts with hydrochloric acid to produce sodium chloride and water.
4A solution of a substance ‘X’ is used for whitewashing. (i) Name ‘X’ and write its formula. (ii) Write its reaction with water.
Answer & Reference:
(i) ‘X’ is calcium oxide (quick lime), CaO.
(ii) CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
(Page 7, Activity 1.4 and In-text Question)
5Why is the amount of gas collected in one test tube double in Activity 1.7? Name the gas.
Answer & Reference:
Water (H₂O) has 2 hydrogen atoms and 1 oxygen atom. During electrolysis, H₂ and O₂ gases are produced in 2:1 ratio. Hence, hydrogen gas volume is double. The gas is hydrogen.
(Page 9, Activity 1.8 and In-text Question)
6Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer & Reference:
Iron displaces copper from copper sulphate (Fe + CuSO₄ → FeSO₄ + Cu). Blue colour fades as Cu²⁺ ions are removed, and iron nail becomes brown due to deposited copper.
(Page 10–11, Activity 1.9)
7Give an example of a double displacement reaction other than in Activity 1.10.
8Identify oxidised and reduced substances: (i) 4Na + O₂ → 2Na₂O (ii) CuO + H₂ → Cu + H₂O
Answer & Reference:
(i) Na is oxidised (gains oxygen), O₂ is reduced.
(ii) H₂ is oxidised (gains oxygen), CuO is reduced (loses oxygen).
(Page 13, In-text Question)
9Which statements about 2PbO + C → 2Pb + CO₂ are incorrect? (a) Lead is reduced. (b) CO₂ is oxidised. (c) Carbon is oxidised. (d) Lead oxide is reduced.
Answer & Reference:
Incorrect: (b) CO₂ is oxidised → false (CO₂ is product, not oxidised).
Correct option: (i) (a) and (b) is wrong; actually (b) alone is incorrect, but per NCERT logic, answer is (i) (a) and (b) is incorrect choice.
However, based on PDF: Carbon is oxidised, PbO is reduced. So (b) is false.
Final answer: (i) (a) and (b) — but note (a) is actually correct. Clarification: Only (b) is incorrect. But as per NCERT key, answer is (i).
(Page 14, Exercise Q1)
10Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is an example of:
Answer & Reference: (d) Displacement reaction — aluminium displaces iron from its oxide.
(Page 15, Exercise Q2)
11What happens when dilute HCl is added to iron filings?
Answer & Reference: (a) Hydrogen gas and iron chloride are produced.
Reaction: Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)
(Page 15, Exercise Q3)
12What is a balanced chemical equation? Why balance it?
Answer & Reference:
A balanced equation has equal number of atoms of each element on both sides. It follows the law of conservation of mass — mass can neither be created nor destroyed.
(Page 3–4, Section 1.1.2)
Answer & Reference:
• Corrosion: Metal reacts with moisture/air (e.g., iron → rust, silver → black Ag₂S). (Page 13)
• Rancidity: Fats/oils oxidise → bad smell/taste. Prevented by nitrogen flushing or antioxidants. (Page 13)